We all know a few types of people when it comes to getting things done: the people who need you to check on them, the ones whose word you can totally trust, and the few who are difficult because they seem to take pride in defying everyone. Well, Gretchen Rubin studied how humans meet expectations and developed the theory of The Four Tendencies. She turned it into a book and in it teaches us how to understand our own, as well as other people's personalities, so that we can all take the path of least resistance in accomplishing goals.